This invention generally relates to electromagnetic tagging systems, and is specifically concerned with a system for remotely detecting and locating polymeric structures of difficult accessibility, such as buried plastic pipes or containers.
Thermoplastic polymers such as polyethylene, polyvinylfluoride, and polypropylene are particularly attractive materials for making buried piping, underground storage containers, and structural geotextiles. These materials are easy to handle, lightweight, relatively inexpensive, easy to join and repair, and (unlike metal pipes and containers) highly resistant to corrosion damage. However, despite these advantages, repair and maintenance of such polymeric structures does become necessary from time to time. And, once buried in the ground or covered with another material, the structures formed from these polymers are virtually impossible to remotely locate by known electronic sensors. Hence, extensive excavation is often necessary to find such structures. The inability to accurately detect the location of such polymer structures can result in accidental damage during the locating excavation. This is a particularly acute problem in the case of polymeric high integrity containers (HIC's) that hold toxic wastes, since accidental damage to these containers resulting from excavation could release radioactive or chemically hazardous materials into the environment.
In the past, a number of techniques have been developed to expedite the location of buried plastic components with particular emphasis on piping. Brightly colored marker ribbons buried above the polymer offer some detection enhancement. However, excavation is required to locate the marker itself and it is possible for the marker and buried component to "drift" over time, resulting in less accurate positioning. Metallic foils and magnetic particle tagged ribbons have also been developed to permit remote electronic location of the markers with metal detectors or eddy current probes. While this approach can minimize exploratory excavation to locate the polymer, the problem of marker drift remains.
Attempts to "mark" the buried component itself so that it becomes visible to electronic detection have included tagging the polymer with embedded magnetic particles, as disclosed in Goodman U.S. Pat. No. 5,051,034, or installing a target wire on the inside of the pipe as disclosed in the Keene U.S. Pat. No. 4,573,829. Unfortunately, the relatively weak detection signal produced by both of these techniques limits the use of these systems to pipes that are buried only at shallow depths. A simple wire conductor provides a relatively small target that does not produce a strong signal unless it is energized with an electric current. However, such energization of course requires access to the wire which complicates the detection operation. When magnetic particles are used, a substantial amount of magnetic material must be added to the polymer in order to produce a strong magnetic signal at typical buried pipe distances of one to two meters. As the target distances and sizes become smaller, concentrations exceeding 50 percent may be required. Such high levels of hard particle additions to thermoplastic alloys can destroy the unique and advantageous mechanical properties that make these materials so attractive for buried component applications. Moreover, when either conductive wires or magnetic particles are used as markers, the signals they generate can be so similar to those generated by adjacent ferromagnetic components (such as iron pipes or steel reinforcing bars) such that a high degree of ambiguity and inaccuracy begins to taint the detection signal.
Clearly, what is needed is a system for tagging underground polymeric structures that provides a strong and unambiguous detection signal, but which does not compromise the advantageous mechanical characteristics of the polymeric material forming the structure. Ideally, the system would be capable of not only accurately detecting the presence of a tagged underground structure at greater distances than the prior art, but of also providing information concerning the orientation, identity, and precise location of the structure despite the close presence of other structures made from ferromagnetic materials. Finally, the system should be inexpensive and simple to implement and convenient to use.